Currently there is an interest in very low power wireless communication enabled devices. The uses of such devices include asset tracking devices, wireless sensors, wireless actuators for applications including asset tracking systems, wireless sensor networks, industrial and environmental monitoring and control systems, wireless personal computer peripherals, toys, security systems, etc.
With the proliferation of wireless communication services there is paucity of available spectrum. Therefore, system designers have turned to the unregulated Instrument, Scientific, and Medical (ISM) bands of 2.45 Ghz and 900 MHz. However, due to the fact that they are unregulated, the likelihood of a wireless system having to contend with high levels of interference must be taken into account.
One class of wireless communication system that is robust in terms of tolerance of interference is spread spectrum communication. One type of spread spectrum communication is Direct Sequence Spread Spectrum (DSSS) communication. DSSS communication systems are in fact able to contend with Signal to Noise Ratios (SNR) of less than unity.
In conventional DSSS communication systems a binary data signal that is biased so that the two signal states have equal and opposite signal levels is multiplied by a higher frequency signal, thereby spreading its spectrum. The higher frequency signal comprises a series of pulses that are polarized according to the values of a series of elements of a DSSS code. In conventional DSSS communication information is encoded by choosing the polarization of a sequence of pulses corresponding to a DSSS codes, according to the value of a corresponding data bit.
One signaling system uses different codes to communicate different N-bit bit patterns thereby achieving a higher data rate. Unfortunately for a given DSSS code length there are only a limited number of DSSS codes that are sufficiently uncorrelated to be used reliably in the same communication system. If the codes are not sufficiently uncorrelated, the robustness of the system to interference can be degraded due to the need to increase threshold levels used in construing a received symbol.
One possible solution is to use a much longer DSSS code length so that there will be more uncorrelated DSSS codes that can be used for higher order Mary signaling in order to achieve higher signal rates. Unfortunately, the processing of longer DSSS codes increases power consumption and reduces battery life. The latter is unacceptable for low power wireless devices that are intended to be sustained in operation for long periods by a battery.
What is needed is spread spectrum signaling method that can achieve higher data rates without having to use longer DSSS codes, or otherwise increasing power consumption.